function myReplace(str, before, after) {
  //return str;
  if(before[0] === before[0].toUpperCase()){
    after = after[0].toUpperCase() + after.slice(1);
  }
  str = str.replace(before,after);
  return str;
}

myReplace("A quick brown fox jumped over the lazy dog", "jumped", "leaped");

function myReplace(str, before, after) {
  //return str;
  var re = /^[A-Z]/;
  if(re.test(before.charAt(0))){
    after = after.charAt(0).toUpperCase() + after.slice(1);
  }
  str = str.replace(before,after);
  return str;
}

myReplace("A quick brown fox jumped over the lazy dog", "jumped", "leaped");